Blood platelets have a finite life span in the peripheral circulation and under normal steady state conditions are removed by a discrete aging process. Earlier studies had suggested that there were structural, metabolic, and functional differences between young and old platelets. Existing methods to isolate platelets for study were inadequate. An efficient method to quantitatively isolate pure blood platelets has been developed in this laboratory. Experiments have shown that there is a strong relationship between platelet density and platelet structure, metabolism and function. Use of a non-human primate model has shown that these density dependent characteristics do correlate with platelet age. Thus, it is now possible to isolate age dependent platelet cohorts and to measure age dependent parameters of pure whole blood platelets. These techniques provide a means to analyze mechanisms of platelet aging to estimate platelet turnover rates, and to isolate platelets of improved quality for potential use in therapeutic transfusion.